1. Field of the Invention
The present invention relates to a projection type image display apparatus such as a projector, etc.
2. Description of Related Art
Conventionally, a projection type image display apparatus uses a liquid crystal panel or a micromirror device as a light modulation device for switching, and displays an image on a screen by projecting a light pattern selected upon controlling light transmission and shielding or polarization.
However, in the abovementioned display, when a liquid crystal panel or a micromirror device is used as a light modulation device, it is necessary that shielded light is removed as unnecessary energy by being absorbed by a polarizer or a light absorbing medium.
In the case of liquid crystals, unnecessary illumination light exists due to the light transmittance, the aperture efficiency of each pixel or accuracy of polarization control. Furthermore, in the case of the micromirror device, it is difficult to efficiently use the pupil of an axially symmetric optical system depending on the aperture efficiency of each pixel and the numerical aperture of a projection lens due to oblique -incidence illumination and the numerical aperture of an illumination system.
Therefore, in order to make a displayed image brighter, a metal halide and a high-pressure mercury lamp is used as a light source, however, a high voltage is required as a light source voltage, so that the light source generates high heat.
Japanese Patent Laid-Open No. H11(1999)-67448 (corresponding to U.S. Pat. No. 6,259,424) and Japanese Patent Laid-open No. 2000-66301 propose method for radically solving this low energy usability. In these two proposals, a luminescent panel (hereinafter, referred to as an organic EL panel) including matrix-arranged organic electroluminescence elements (hereinafter, referred to as organic EL elements) is used, the respective organic EL elements of this organic EL panel are driven to emit light based on image signal, and the projection optical system projects and displays an image on a display target object. The organic EL elements require no illumination light sources separately since they are self-emitting devices, and no transmission type liquid crystal panel is required since the organic EL panel emits light including image information. Therefore, obtained light can be efficiently utilized for display. Accordingly, without generating unnecessary light energy, high-luminance display can be easily achieved at a low electric power, and an image can be outputted by only the organic EL panel, so that the apparatus structure becomes simple and can be easily reduced in size and weight.
However, after the organic EL elements continuously emit light at a high luminance, luminance greatly lowers. One of the causes of this problem is that heat is generated due to a current supplied for driving the organic EL elements, accumulates, and raises the element temperature, and this deteriorates the structure and characteristics of the organic thin films and gradually lowers luminous efficiency.
On the other hand, as a technique for suppressing luminance lowering of organic EL elements, pulse intermittent turning-on drive of organic EL elements is disclosed in Japanese Patent Laid-Open No. H07(1995)-230880. With this technique, deterioration of luminous efficiency of organic EL elements is considerably suppressed.
However, in a case where images are displayed by, for example, 60 frames per second by means of pulse intermittent turning-on drive of organic EL elements, when a dark image with a small turning-on pulse time duty is displayed, flickering is observed due to intermittent emission. Furthermore, when emission pixels of R, G, and B that are three primary colors or organic EL panels are made to simultaneously emit light, the amplitude of the power supply increases, so that allowance must be given for the power to be supplied or the power source capacity must be increased.
Furthermore, a possible cause of efficiency lowering of the organic EL elements is as follows. That is, an organic EL material itself changes its chemical structure into anion or cation excitons and repeats light energy emission according to potential energy displacement, and furthermore, an organic electroluminescent layer is composed by using a fluorescent or phosphorescent material as a luminous body and a material for dispersing the abovementiond material and transporting electronic charge carriers to the luminous bodies, so that the probability that the organic EL material that changes its chemical structure or causes particle aggregation to emit light changes into a form other than the desired change form does not theoretically become 0%.
The speed of changing into a form other than the desired change form is influenced by state stability of the luminescent material, combination of an electronic charge carrier transporting material, etc., that serves as a binder and environmental medium materials, applied electric intensity, hydrolysis according to the moisture environment, and defect potential generation, etc. Among these, as a main cause of deterioration concerning the speed of chemical reaction, there is deterioration acceleration due to a self-raising temperature parameter according to heat energy generation in a photoelectric converting process.
This deterioration acceleration roughly follows the Arrenius equation. Therefore, when emission driving is carried out in a low-power supplied condition at a low temperature, the speed of changing the photoelectric conversion efficiency slows and lengthens the life. However, as the power to be supplied is increased to increase the emission intensity, the photoelectric conversion efficiency exponentially lowers, resulting in a short life of the element. For example, on the assumption that the quality guarantee period of normal commercial-off-the-shelf products is one year, stable emission for several thousands to tens of thousands of hours is required. Therefore, nonconformity occurs in that the quality stability lowers as it is attempted to achieve brighter and higher-quality image display.
Furthermore, a reduction in deterioration speed of the photoelectric conversion efficiency of the organic EL material can be achieved by cooling the EL elements themselves to a low temperature, however, it is also influenced by the outside humidity in a case where the cooling temperature is maintained at a temperature lower than the outside air temperature. For example, in a high-temperature high-humidity environment, for example, at a temperature of 30° C. and a humidity of 80%, water vapor condenses into dew condensation on the EL elements, and when waterdrops are generated in the light emission direction, it becomes impossible that an image to be displayed by the EL elements is projected as it is by a projection lens, resulting in a distorted image being projected.
In addition, as a result of forcibly cooling the organic EL elements, cooling energy is required separately, and this spoils the advantage of a reduction in energy consumption that is the original intent of using the organic EL elements.